In the conventional kraft pulping process, the active pulping chemicals are sodium hydroxide (NaOH) and sodium sulphide (Na2S). The amount of Na2S relative to the amount of NaOH is characterized by a parameter termed the sulphidity which is defined as follows:Sulphidity (%)=mNa2S×100/(mNaOH/2+mNa2S),where mNa2S the number of moles of Na2S and mNaOH is the number of moles of NaOH
In the conventional kraft pulping process, the sulphidity of the pulping liquor is typically in the range 25-40%. In kraft-type pulping, increasing the sulphidity of the pulping liquor is usually beneficial from the point of view of the pulping stage. Typically, the upper limit on sulphidity in the conventional kraft pulping process is not set by the demands of the pulping stage but by the demands of the chemical-recovery process. When the sulphidity exceeds a certain value, sulphur dioxide (SO2) emissions from the chemical-recovery boiler increase to an unacceptable level, all other process variables being unchanged. The increased SO2 emission level is a consequence of the fact that the release of alkali-metal compounds from the spent pulping liquor during combustion is no longer sufficient for the capture of the greater part of the sulphur compounds released from the liquor.
Kraft-type pulping at very high sulphidity is a known pulping method. In fact, the most well-known specific method employs 100% sulphidity. In other words, in this particular method, only one active pulping chemical—Na2S—is employed. This method, which was studied and developed in the late 1960s and early 1970s, goes by the name of the Alkafide process (Munk L., Todorski Z., Bryce J. R. G., Tomlinson G. H., Pulp Paper Mag. Can. 65(1964)10, p. T411; Tomlinson G. H., Canadian Patent 725,072; Tomlinson G. H., U.S. Pat. No. 3,347,739; Ingruber O. V., Allard G. A., Pulp Paper Mag. Can. 74(1973)11, p. T354). According to these previous studies, the alkali consumption is reduced by 30-40% in Alkafide pulping compared to conventional kraft pulping. The pulp yield is reported to be the same for kraft and Alkafide pulps, while the strength properties are improved by the higher sulphidity. In other words, from the point of view of the pulping stage only, pulping at 100% sulphidity is superior to conventional kraft pulping. However, widespread commercialization of this pulping method never occurred. Presumably this was due to the lack of a cost-effective method for recovering the pulping chemical, Na2S.
In present-day pulp mills based on conventional alkaline pulping processes, such as the conventional kraft pulping process, only limited amounts of by-products are recoverable in an economically viable way. These potential by-products—turpentine and tall oil—originate from the extractives component of the pulping raw material. However, the spent pulping liquor contains large quantities of other potential by-products originating from the pulping raw material. These include lignin and aliphatic hydroxy acids. In present-day alkaline pulping mills, these components are exploited as fuel in the chemical-recovery boiler. However, in recent years, interest in recovering additional by-products from spent alkaline pulping liquors has been increasing. The greatest techno-economical challenge is associated with the need to lower the pH of the spent pulping liquor in order to liberate organic compounds from their sodium salts. Utilization of purchased acid to achieve this is not an attractive option because of both the direct costs of the acid and the possible indirect costs arising from disturbances to the mill chemical balances. Ideally, the required acidification of the spent pulping liquor would be carried out with internally generated acid.
In pulp mills employing conventional alkaline pulping processes, such as the conventional kraft pulping process, potentially problematic non-process elements include silicon and phosphorus. These accumulate in the lime cycle of the mill and have a severe deleterious impact on the operability and efficiency of that cycle. (The lime cycle provides calcium oxide (CaO) for reactions in the main recovery cycle, accepts the reaction product, calcium carbonate (CaCO3), and reconverts the CaCO3 into CaO.) In addition, silicon compounds dissolved in the spent pulping liquor cause problems during concentration of the liquor by evaporation (higher viscosity, deposits) and combustion of the liquor (deposits). The severity of the silicon problem obviously increases with increasing content of silicon in the raw material employed for pulping. Cereal straws and certain tropical woods have high silicon contents. Silicon may be effectively removed from chemical-recovery cycle by lowering the pH of the spent pulping liquor and removing the silicon containing material thus precipitated. As in the case of recovery of by-products such as lignin, the required acidification of the spent pulping liquor would, ideally, be carried out with internally generated acid.
Use of internally generated acid has previously been proposed for acidification of spent pulping liquor. The main emphasis has been on the exploitation of carbon dioxide (CO2) contained in flue gases. CO2, a weak acid, is effective in lowering the pH of spent alkaline pulping liquor to around 10, which is sufficient to precipitate a significant amount of the lignin contained in the liquor, thus allowing recovery of lignin as a by-product. Similarly, several known methods for purging silicon from the chemical-recovery cycle are based on the use of CO2 for acidifying the spent pulping liquor. One approach has been to use flue gas as such as the acidifying medium. This approach has not led to any long-lived commercial applications. Another approach is to remove CO2 from flue gases and use the recovered CO2 in a concentrated form. This approach has proved too costly. Use of purchased CO2 for acidifying spent pulping liquor to a pH around 10 is the basis of several current processes for recovering lignin from spent pulping liquor.
At a conventional kraft pulping mill, one stream that is readily convertible into acid is the stream made up of concentrated non-condensable gases (CNCG) collected as a side-product from several mill operations, in particular from pulping and evaporation operations. Sulphur containing compounds, in particular hydrogen sulphide (H2S), methyl mercaptan (CH3SH) and dimethyl sulphide ((CH3)2S), are main components in these gases. Oxidation of these gases yields an acidic compound, sulphur dioxide (SO2), which may be further converted into the strong mineral acid, sulphuric acid (H2SO4). However, the amount of acid that could be produced in this way is relatively small, which may explain why acid generated from CNCG has not, in general, been proposed for acidifying spent kraft pulping liquor. Typically, the amount of sulphur contained in the total CNCG stream of the pulp mill could provide enough H2SO4 to acidify less than 5% of the total spent pulping liquor to a pH of 10. In a method disclosed in US Patent Application US2008/0214796A1, acid generated from CNCG is used for washing lignin precipitated from spent kraft pulping liquor, while CO2 is employed for the preceding acidification step.
In a method disclosed in Patent Application WO2010/143997A1, gases, mainly CO2 and H2S, are recycled from the acidic washing stage of a lignin-recovery process to the precipitation stage of the same lignin-recovery process. Being acidic gases, the recycled CO2 and H2S can reduce, to some extent, the amount of external acid, typically CO2, employed to acidify spent pulping liquor in the precipitation stage. In one of the embodiments of the method, the recycled H2S is first converted into stronger acid such as H2SO4. It is important to note that (1) a very minor or negligible amount of H2S is released in the acidification stage of this method, (2) in the example given in the patent document, a significant part of the savings in acid consumption in the precipitation stage is attributable to recycled CO2 rather than to recycled H2S and (3) the amount of input acid required in the acidic washing stage—measured in terms of amount of H+ ions—clearly exceeds the amount of acid that could be supplied by utilizing or converting all the CO2 and H2S released in the same acidic washing stage. Thus, the amount of H2S recycled in this method is much less than the amount that would be necessary to cover all the acid consumed in the process even if the H2S were to be first converted to a stronger acid such as H2SO4.
In the light of the prior art, there is a clear need for:                1. a technically and economically viable method for recovering pulping chemicals in conjunction with kraft-type pulping at very high sulphidity, and        2. a technically and economically viable method for internally generating, on a large scale, acid for lowering the pH of spent alkaline pulping liquor and thus facilitating the recovery of by-products and/or the removal of certain non-process elements from the liquor.        
An object of the present invention is to provide a method which can meet both these needs simultaneously.